The use of air dropped munitions in warfare is well known. Such munitions typically comprise a generally tubular housing which is substantially filled with an explosive, a nose having an impact fuse and detonator for detonating the explosive, and fins for stabilizing the munition during its decent, such that it impacts the target with the nose thereof so as to assume proper detonation. The use of such air dropped munitions has provided a substantial advance in the art of warfare by facilitating the destruction of enemy targets while mitigating undesirable loss of life and/or destruction of military equipment.
However, as those skilled in the art will appreciate, conventional munitions must generally either be released with very high accuracy or in very large numbers in order to effectively destroy a desired target. Thus, it is frequently necessary to either drop such munitions from an undesirably low altitude or to fly an undesirably large number of sorties. Dropping conventional munitions from a lower than desirable altitude exposes the aircraft and crew to hazardous anti-aircraft artillery and ground-to-air missiles. The accuracy of such anti-aircraft artillery and ground-to-air missiles is substantially enhanced by the reduced range to target (altitude of the aircraft) provided by such low flight. For this reason low altitude bombing is extremely dangerous and is very rarely performed. Of course, flying an undesirably large number of sorties is expensive, time consuming, and exposes the aircraft and crew repeatedly to air defense weaponry such as anti-aircraft artillery and ground-to-air missiles.
Thus, in order to compensate for the lack of accuracy inherent when such conventional munitions are dropped from sufficient altitude so as to substantially mitigate the effectiveness of anti-aircraft artillery and/or ground launched missiles, a higher number of sorties must necessarily be flown. This is done in order to effect the delivery of a larger number of such conventional munitions to the target, thereby compensating for the reduced accuracy of such high altitude bombing and consequently enhancing the likelihood that the target will be destroyed.
In an attempt to overcome the deficiencies of conventional munitions in reliably destroying ground targets, particularly when dropped from a high altitude, smart munitions have been developed. Such smart munitions utilize a guidance and flight control system to maneuver the munition to the desired target. The guidance system provides a control signal to the control surfaces based upon the present position of a munition and the position of the target, so that the control surfaces maneuver the munition toward the target. Such guidance systems operate according to well known principles and typically utilize such technologies as laser guidance, infrared guidance and/or radar. Such guided munitions thus facilitate the reliable destruction of enemy targets by an aircraft flying at a sufficient altitude to substantially mitigate the effectiveness of anti-aircraft artillery and/or ground launch missiles and do so with a substantially reduced number of sorties.
It is also known to use the global positioning system (GPS) to guide various different munitions to intended targets. Multiple GPS satellite constellations are in the process of being implemented or are in the planning stage to provide accurate navigational information and position fixes for appropriate receivers or stations anywhere on the surface of the Earth. Among these GPS systems are the U.S. government-operated Navigation Satellite Timing and Ranging Global Positioning System, "NAVSTAR GPS", the "GLONASS" system planned by the government of the former Soviet Union, and two European systems known as "NAVSAT" and "GRANAS" presently under development. For ease of description, the following discussion and disclosure will focus specifically on the features of and use with the NAVSTAR GPS, although it will be understood that the invention has equal applicability to other global positioning systems.
The U.S. government-operated NAVSTAR GPS is designed to have four orbiting GPS satellites existing in each of six separate circular orbits to constitute a total of twenty-four GPS satellites, with twenty-one being operational and three serving as spares. The satellite orbits are neither polar nor equatorial but lie in mutually inclined planes and each satellite orbits the Earth approximately once every 12 hours, completing exactly two orbits while the Earth turns one revolution. With this arrangement at least four satellites come within the same field of view twenty-four hours a day all around the world. The position of each satellite at any given time is precisely known and navigation signals are continuously transmitted to the Earth providing position information indicating the position of the satellite in space with respect to time (GPS time). This position information is known as ephemeris data. In addition to the ephemeris data, the navigation signals transmitted by each satellite include an indication of the precise time at which the signal was transmitted. Consequently, the distance or range between a navigation signal receiver and a transmitting satellite may be determined using this time indication: by 1) noting the time at which the signal was received at the receiver, 2) calculating the propagation time delay, i.e., the difference between the time transmitted and time received, and 3) multiplying the delay by the speed of propagation of the signal. The result of this determination will yield a "pseudorange" from the transmitting satellite to the receiver. The range is called a "pseudorange" because inaccuracies may occur due to such factors as the receiver clock not being precisely synchronized to GPS time, and delays introduced into the navigation signal propagation times by its propagation through the atmosphere. These inaccuracies result, respectively, in a clock bias (error) and an atmospheric bias (error), with clock biases possibly as large as several milliseconds. In any event, using the two pieces of information in a navigation signal, i.e., the ephemeris data and the pseudorange, from at least four satellites, the position and time of a receiver with respect to the center of the Earth can be determined using passive triangulation techniques.
A more detailed discussion on the NAVSTAR GPS is found in an article by B. W. Parkinson and S. W. Gilbert, entitled, "NAVISTAR: Global Positioning System--Ten Years Later," Proceedings of the IEEE, Vol. 71, No. 10, October 1983, and in a text "GPS: A Guide to the Next Utility", published by Trimble Navigation Ltd., Sunnyvale, Calif., 1989, pp. 1-47, both of which are incorporated herein by reference.
The NAVSTAR GPS envisions two types of code modulation for the carrier wave propagating pseudorandom signals. In the first type, "Coarse/Acquisition" code, the carrier is modulated by a "C/A signal", and referred to as the C/A code and also as the "Standard Positioning Service" (SPS). The second type of modulation is commonly referred to as the "precise" or "protected" (P) code, and also as the "Precise Positioning Service" (PPS). An encrypted version of the P-code, i.e., Y code, is intended for use only by Earth receivers specifically authorized by the U.S. government so that Y-code sequences are kept secret and not made publicly available. This forces more NAVSTAR GPS users to rely solely on the data provided via C/A code modulation, which unfortunately results in a less accurate positioning system. Moreover, the U.S. government selectively corrupts the GPS data by introducing errors into the C/A code GPS signals being transmitted from the GPS satellites by changing the clock parameters, that is, the clock parameters for one or more satellites may be slightly or substantially modified, such as by the intentional dithering of the phase and frequency of the satellite clock, which practice is known as "selective availability" or simply SA. SA may be activated for a variety of reasons, e.g., the Department of Defense may activate it for national security. When SA is activated, the U.S. government is still able to use the NAVSTAR GPS because it has access to the means of compensation to remove SA effects. The uncompensated C/A code data, however, may be rendered substantially less accurate, i.e., degraded. In view of the foregoing distinction, C/A code modulation receivers are referred to as "civil or civilian" receivers or sets, and Y-code modulation receivers are referred to as military receivers or sets. For purposes of generality, reference to a "civil" receiver herein will indicate a varying or degraded accuracy GPS receiver or set, and to a "military" receiver will indicate a precise accuracy GPS receiver or set.
In many applications of GPS, it is desirably to use a civil GPS receiver, such as in a mobile or expendable vehicle, to reduce cost and complexity, even though the user has access to the crypto keys that afford Precise Positioning Service (PPS) accuracy. This is especially true in expendable vehicles or weapons for launching from some form of launch vehicle. If the user is authorized for PPS, he may have a military GPS receiver on the launch vehicle that is much more accurate than the civil set in the expendable one.
A Differential GPS (DGPS) correction process is available for high-accuracy civil GPS applications, that uses fixed surveyed antenna coordinates and requires the corrections to come from a ground station almost in real time. Typical civil GPS receivers have inherent accuracy limitations, largely due to the aforementioned intentional degradation introduced into the signals by the U.S. Department of Defense, which limitations can be removed by the DGPS process. Civil GPS receiver accuracy, which as noted above is commonly referred to as Standard Positioning Service or SPS, without differential correction is specified to be better than 100 meters. While some of the accuracy limitation is due to the use of only one frequency, which prevents the measurement of errors due to the Ionosphere delays, and the use of the Coarse/Acquisition (C/A) code instead of the more accurate Precision P(Y) code, the primary contributor to the accuracy limitation is the intentional dithering of the phase and frequency of the satellite clock, i.e., SA. A military set removes the SA errors with an algorithm that uses data which is only available to authorized users through the use of crypto keys. In addition, the user of a militarized receiver can track a second frequency which allows the measurement of and compensation for the Ionosphere delays.
Although such guided munitions are very accurate and thus, extremely effective in destroying enemy targets upon the ground when dropped from high altitude, they do possess the inherent deficiency of being extremely expensive. Not only is the guided munition itself substantially more expensive than conventional, i.e., non-guided, munitions, but the aircraft delivering the munition must undergo extensive and very expensive modifications in order to accommodate the guided munition. Typically, additional avionics and control equipment is required in the cockpit, wiring must be provided from the cockpit to the bomb mount or pylon, and a weapon specific electrical and mechanical interface to the guided munition must be provided. Because they are expensive, such guided munitions are typically reserved for use only against targets having a very high strategic value.
In view of the foregoing it would be desirable to provide a munitions, e.g., an air dropped munitions, having enhanced accuracy with respect to conventional munitions, but being far less expensive than guided munitions.